1. Field of the Invention.
The present invention relates to a method in evaporating liquid and comprises evaporation of liquid from a solution or suspension (whereafter the residue may remain in liquid form or consist of a substantially dry substance), as well as cooling of a warm gas by atomizing or spraying a liquid (normally water) in the same. Thus, the present invention comprises as well so-called "spray concentrating" as "spray drying" and "conditioning", i.e. cooling and/or moistening, of flue gas or combustion gas.
2. Description of the Prior Art.
It is known to evaporate liquid from a suspension or solution by spraying the same into an evaporating chamber, a flow of warm drying gas being simultaneously directed into said evaporating chamber. The amount of vapour which may be absorbed and removed by the drying gas passing through the evaporating chamber is -- for a certain moisture content of the drying gas -- dependent on the inlet temperature of the gas. Therefore, in order to obtain a high evaporating capacity it is desired to obtain a relatively high temperature of the drying gas being directed into the evaporating chamber. Consequently, it has been proposed to use warm combustion gas from an oil burner as drying gas which is passed directly from the burner to the evaporating chamber. However, the temperature of the drying gas may then be so high that special measures must be taken in order to protect the inner walls of the evaporating chamber. For example, these walls may be provided with a lining of firebricks or another fireproof material. This is, however, a relatively expensive solution. Furthermore, in U.S. Pat. No. 2,818,917 it has been proposed to reduce the temperature of the combustion gas from the burner by mixing the gas with cold atmospheric air before the gas is introduced into the evaporating chamber. By the last-mentioned solution the high evaporating capacity desired is, however, not obtained.
When using high inlet temperatures for the drying gas problems are often encountered due to the fact that dried solid constituents of the suspension or solution are deposited on the inner walls of the evaporating chamber so that the evaporation process must be interrupted from time to time in order to clean the chamber walls. This is true even when the evaporating process is a spray concentrating process so that the suspension or solution remains in liquid form after treatment in the evaporating chamber.
U.S. Pat. No. 2,559,989 discloses a spray drying and spray concentrating apparatus comprising an evaporating chamber into which drying gas is continuously introduced through an inlet tube provided at its free end with gas distributing means adapted to secure a diffuse flow of drying gas into the evaporating chamber. Such gas distributing means are rather heat sensitive and may become damaged if the temperature of the drying gas is relatively high, for example when combustion gas is used as drying gas. Furthermore, if the drying gas, such as waste gases, contains soot particles or other solid particles such particles are liable to form deposits on the distribution means. Therefore, the distribution means must be cleaned from time to time.